Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in a cryoline-based molten electrolyte at temperatures around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode blocks forming the cell bottom floor. The cathode blocks are usually made of an anthracite based prebaked carbon material containing coal tar pitch as a binder joined with a ramming paste mixture of anthracite, coke, and coal tar.
In Hall-Heroult cells, a molten aluminium pool above the carbon blocks acts as the cathode where the reduction to aluminium takes place. The carbon lining or cathode material has a normal useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation by sodium, which causes swelling and deformation of the cathode carbon blocks and ramming paste. In additon, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode blocks beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact and an excessive iron content in the aluminium metal produced.
Extensive research has been carried out with Refractory Hard Metals (RHM) such as TiB.sub.2 as cathode materials. TiB.sub.2 and other RHM's are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool. Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the saving of energy by reducing the anode-cathode distance.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminium production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
Various types of TiB.sub.2 or RHM layers applied to carbon substrates have failed due to poor adherence and to differences in thermal expansion coefficients between the titanium diboride material and the carbon cathode block.
U.S. Pat. No. 3,400,061 describes a cell without an aluminium pool but with a drained cathode of Refractory Hard Metal which consists of a mixture of Refractory Hard Metal, at least 5 percent carbon, and 10 to 20% by weight of pitch binder, baked at 900.degree. C. or more and rammed into place in the cell bottom. Such composite cathodes have found no commercial use probably due to susceptibility to attack by the electrolytic bath.
U.S. Pat. No. 3,661,736 claims a composite drained cathode for an aluminium production cell, comprising particles or pieces of arc-melted "RHM alloy" embedded in an electrically conductive matrix of carbon or graphite and a particulate filler such as aluminium carbide, titanium carbide or titanium nitride. However, in operation, grain boundaries and the carbon or graphite matrix are attacked by electrolyte and/or aluminium, leading to rapid destruction of the cathode.
U.S. Pat. No. 4,308,114 discloses a cathode surface of RHM in a graphitic matrix made by mixing the RHM with a pitch binder and graphitizating at 2350.degree. C. or above. Such cathodes are subject to early failure due to rapid ablation, and possible intercalation by sodium and erosion of the graphite matrix.
U.S. Pat. No. 4,466,996 proposed applying a coating composition comprising a pre-formed particulate RHM, such as TiB.sub.2, a thermosetting binder, a carbonaceous filler and carbonaceous additives to a carbonaceous cathode substrate, followed by curing and carbonisation. But it was still not possible by this method to obtain coatings of satisfactory adherence that could withstand the operating conditions in an aluminium production cell.
U.S. Pat. No. 4,595,545 discloses the production of titanium diboride or a mixture thereof with a carbide and/or a nitride of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum or tungsten by carbothermic, carbo-aluminothermic or alumino-thermic reaction, under vacuum or an inert atmosphere, of a glass or microcristalline gel of oxide reactants prepared from organic alkoxide precursors. This glass or gel was then ground and formed into bodies and sintered into bodies of titanium diboride/alumina-based materials as components of aluminium production cells. But such sintered materials are subject to attack and grain-boundary corrosion when in contact with molten aluminium. Furthermore, the method was not suitable for producing large pieces such as blocks for use as cathodes in aluminium production cells.
The use of self-propagating combustion synthesis (also called micropyretic reaction) to produce net shaped ceramic electrodes for use in aluminium production has been described in WO 92/13977 and WO 92/22682, wherein a particulate combustion mixture for producing a ceramic or metal-ceramic composite was mixed with particulate fillers and inorganic binders. None of these materials contained carbon.
U.S. patent application Ser. No. 861,513 (now U.S. Pat. No. 5,310,476), the contents whereof are incorporated herein by way of reference, proposed producing a protective refractory coating on a substrate of carbonaceous or other material as component in an aluminium production cell, by applying to the substrate a micropyretic reaction layer from a slurry containing particulate reactants in a colloidal carrier, and initiating a micropyretic reaction. The colloidal carrier was at least one of colloidal alumina, colloidal silica, colloidal yttria and colloidal monoaluminium phosphate.
U.S. patent application Ser. No. 07/897,726, the contents whereof are incorporated herein by way of reference, proposed a carbon containing paste for use in particular as components of electrolytic cells as such or compacted to form anodes, cathodes and cell linings of cells for the electrolysis of alumina for the production of aluminium in Hall-Heroult cells. The paste consisted essentially of a compact mixture of one or more particulate carbonaceous material(s) with a non-carbonaceous non-polluting binder and optionally with one or more fillers, the binder being a suspension of one or more colloids or being derived from one or more colloid precursors, colloid reagents or chelating agents.
To date, no carbon-based composite material containing a refractory hard metal boride, carbide or borocarbide has proven satisfactory for use as component of aluminium production cells. Such materials have been expensive to produce, and it has been difficult to produce the materials in large pieces serviceable in aluminium production cells. Moreover, the resistance of such materials to attack by melt components has been unsatisfactory.